Ionic iron(III) complexes bearing a dialkylbenzimidazolium cation: Efficient catalysts for magnesium-mediated cross-couplings of aryl phosphates with alkyl bromides

Article abstract of DOI:10.1002/aoc.3671

A series of ionic iron(III) complexes of general formula [HLⁿ][FeX₄] (HL¹?=?1,3-dibenzylbenzimidazolium cation, X?=?Cl, 1; HL¹, X?=?Br, 2; HL²?=?1,3-dibutylbenzimidazolium cation, X?=?Br, 3; HL³?=?1,3-bis(diphenylmethyl)benzimidazolium cation, X?=?Br, 4) were easily prepared in high yields by the direct reaction of FeX₃ with 1 equiv. of [HLⁿ]X under mild conditions. All of them were characterized using elemental analysis, Raman spectroscopy and electrospray ionization mass spectrometry, and X-ray crystallography for 1 and 4. In the presence of magnesium turnings and LiCl, these air- and moisture-insensitive complexes showed high catalytic activities in direct cross-couplings of aryl phosphates with primary and secondary alkyl bromides with broad substrate scope, wherein complex 4 was the most effective.

Full text of DOI:10.1002/aoc.3671

Received: 24 August 2016
DOI 10.1002/aoc.3671
Revised: 26 September 2016
Accepted: 27 September 2016
F U L L PA P E R
Ionic iron(III) complexes bearing a dialkylbenzimidazolium
cation: Efficient catalysts for magnesium‐mediated cross‐couplings
of aryl phosphates with alkyl bromides
Zhuang Li | Bing Lu | Hongmei Sun | Qi Shen | Yong Zhang
Key Laboratory of Organic Synthesis of Jiangsu
n
1
A series of ionic iron(III) complexes of general formula [HL ][FeX ] (HL = 1,3‐
Province, College of Chemistry, Chemical
Engineering and Materials Science, Soochow
University, Suzhou 215123, People's Republic of
China
4
1
2
dibenzylbenzimidazolium cation, X = Cl, 1; HL , X = Br, 2; HL = 1,3‐
dibutylbenzimidazolium cation, X = Br, 3; HL = 1,3‐bis(diphenylmethyl)
benzimidazolium cation, X = Br, 4) were easily prepared in high yields by the direct
reaction of FeX with 1 equiv. of [HL ]X under mild conditions. All of them were
characterized using elemental analysis, Raman spectroscopy and electrospray ioniza-
tion mass spectrometry, and X‐ray crystallography for 1 and 4. In the presence of
magnesium turnings and LiCl, these air‐ and moisture‐insensitive complexes showed
high catalytic activities in direct cross‐couplings of aryl phosphates with primary
and secondary alkyl bromides with broad substrate scope, wherein complex 4 was
the most effective.
3
Correspondence
n
Hongmei Sun, Key Laboratory of Organic
Synthesis of Jiangsu Province, College of
Chemistry, Chemical Engineering and Materials
Science, Soochow University, Suzhou 215123,
People's Republic of China.
3
Email: sunhm@suda.edu.cn
Funding information
National Natural Science Foundation of China,
Grant/Award Number: 21472134. 21172164.
KEYWORDS
benzimidazolium salt, iron(III) complex, magnesium‐mediated cross‐coupling,
phosphate
[
5]
1
|
INTRODUCTION
reagents. Compared with conventional cross‐coupling reac-
tions that involve the direct formation of Grignard reagents,
this new reaction pattern exhibits lower hazard potential,
safer and simpler operation and cost savings. However, inves-
tigations into this new protocol are in their infancy. The cou-
pling partners most intensively explored for these reactions
[
1]
Since the original work of Tamura and Kochi, transition‐
metal‐catalyzed Grignard cross‐coupling reactions have
evolved into a powerful and versatile synthetic tool for
[2]
forming C─C bonds. Over the past decade considerable
efforts have been made to develop iron‐catalyzed Grignard
cross‐couplings as alternative or complementary methods to
traditional palladium‐ and nickel‐catalyzed methods for
forming C─C bonds. The advantages of using iron include
its environmental friendliness, low toxicity and low cost, as
well as the unique activities of iron‐based catalysts in
Grignard reactions. For example, iron complexes have an
ability to efficiently suppress β‐hydrogen elimination in
cross‐couplings of both primary and secondary alkyl halide
[
5]
[5]
have been aryl bromides, alkenyl bromides, alkyl bro-
[
5]
[5]
mides and, recently, inert alkyl chlorides. It would be
interesting to expand the scope of this methodology to O‐
based substrates, particularly phenolic derivatives. These
substrates are more easily available and more environmen-
[3]
[
6]
tally friendly than the corresponding halides.
The search for novel coupling partners and efficient cata-
lysts continues to generate great interest in iron catalysis.
Recently, several kinds of O‐based electrophiles, such as
[4]
[7]
[7]
substrates with aryl Grignard reagents.
aryl/alkenyl triflates, aryl/alkenyl tosylates, aryl/alkenyl
[
7]
[7]
[7]
One notable advance in this field is an iron‐catalyzed
direct cross‐coupling of two organic halides in the presence
of magnesium turnings. This reaction features in situ gene-
ration of Grignard reagents, and thus removes the need for
pre‐synthesis of inherently moisture‐sensitive Grignard
pivalates, 2‐pyrone derivatives, aryl carbamates, aryl
[7] [7]
sulfamates and alkenyl acetates, have been shown to cou-
ple effectively with Grignard reagents. However, there are
only a few reports of the use of phosphates in iron‐catalyzed
Grignard cross‐coupling reactions. In fact, phosphates
Appl Organometal Chem 2016; 1–7
wileyonlinelibrary.com/journal/aoc
Copyright © 2016 John Wiley & Sons, Ltd.
1

2
LI ET AL.
represent an attractive alternative electrophile not only
because of their vast abundance in living organisms, but also
because phosphates have simple and inexpensive preparation
methods. Furthermore, phosphates are even more environ-
analysis was performed using a Varian CP‐3800 instrument
equipped with a flame ionization detector and an AT.OV‐
101 capillary column (30 m × 0.32 mm inner diameter,
0.10 μm film). The oven temperature was held at 80 °C for
[
8]
−1
mentally benign and stable than phenolic derivatives.
2 min, increased to 280 °C at 10 °C min and held for
Begtrup and co‐workers first reported the alkylation of
alkenyl phosphates by BuMgCl in the presence of Fe(acac)
2 min. High‐resolution mass spectra were obtained using a
GCT‐TOF instrument with electrospray ionization (ESI) or
chemical ionization source.
n
(
acac = acetylacetonato); however, only two moderate‐
3
[9]
yielding examples were described. Gager and co‐workers
showed the utility of Fe(acac) in the couplings of alkenyl
or terminal conjugated dienyl phosphates, while Quan
[
9]
3
2
.2 | Preparation of complexes
[9]
1
2
.2.1
|
[HL ][FeCl ] (1): procedure A
4
and co‐workers used Fe(acac) for the coupling of
3
2
[14]
[9]
Following a procedure similar to that for [HL ][FeCl ],
a
pyrimidin‐2‐yl phosphates.
However, aryl phosphates,
4
1
Schlenk flask was charged with [HL ]Cl (0.61 g, 1.80 mmol),
tetrahydrofuran (THF; 40.0 ml) and a stirring bar. To this sus-
which are derived from unactivated phenols, remain chal-
lenging substrates because they possess a less reactive aro-
[6]
pension was added a THF (10 ml) solution of FeCl (0.29 g,
matic C─O bond than alkenyl phosphates. Furthermore,
to the best of our knowledge, iron‐catalyzed magnesium‐
mediated cross‐coupling reactions involving aryl phosphates
have yet to be explored.
3
1
.80 mmol). The reaction mixture was stirred for 2 h at room
temperature, during which time the mixture became a homo-
geneous yellow‐green solution. The reaction solution was fil-
tered and evaporated to dryness. The residue was extracted
with THF (3 × 10.0 ml), and crystallized from THF–hexane
solution at 0 °C. The product 1 was precipitated as yellow‐
green crystals in a yield of 87% (0.77 g), m.p. 175 °C. Anal.
Calcd for C H Cl FeN (%): C, 50.75; H, 3.85; N, 5.64.
As a part of our ongoing studies concerning the utility of
imidazolium salts to develop efficient iron‐based catalysts
[10]
for cross‐coupling reactions, we report here a facile synthe-
sis of a series of ionic iron(III) complexes (1–4). We performed
X‐ray structural characterization of the complexes and applied
them as efficient catalysts for a direct cross‐coupling between
aryl phosphates and primary or secondary alkyl bromides with
magnesium turnings. Dialkylbenzimidazolium salts have
already been used as ligands in a variety of palladium‐ and
nickel‐based catalytic systems developed for cross‐coupling
21 19
4
2
Found (%): C, 50.76; H, 3.84; N, 5.65. MS (ESI+): m/z
+
−1
2
99.15 [C H N ] (100%). Raman spectrum: 334 cm
21 19 2
−
([FeCl ] ).
4
1
2
4
.2.2 | [HL ][FeCl ] (1): procedure B
[
11]
1
reactions. However, their potential utility in iron‐based cat-
A Schlenk flask was charged with [HL ]Cl (0.61 g,
1.80 mmol), THF (40.0 ml) and a stirring bar. To this suspen-
sion was added a THF (10 ml) suspension of FeCl ⋅6H O
[3,10,12]
alytic systems remains poorly explored.
3
2
(0.39 g, 1.80 mmol). The reaction mixture was stirred for
2
h at room temperature, during which time the mixture
2
2
|
EXPERIMENTAL
became a homogeneous yellow‐green solution. The reaction
solution was filtered and evaporated to dryness. The residue
was extracted with THF (3 × 10.0 ml), and crystallized from
THF–hexane solution at 0 °C. The product 1 was precipitated
as yellow‐green crystals in a yield of 85% (0.75 g).
.1 | General
All manipulations were performed under pure argon with
exclusion of air and moisture using standard Schlenk tech-
niques. Solvents were distilled from Na/benzophenone ketyl
under pure argon prior to use. All substrates, reagents and
materials were purchased from Sigma Aldrich, Alfa Aesar
and TCI Chemicals. 1,3‐Dibenzylbenzimidazolium chloride
1
2
.2.3
|
[HL ][FeBr
4
] (2)
1
A Schlenk flask was charged with [HL ]Br (0.72 g,
1.90 mmol), THF (40.0 ml) and a stirring bar. To this suspen-
sion was added a THF (10 ml) solution of FeBr (0.56 g,
1
[13]
2
([HL ]Cl),
1,3‐dibutylbenzimidazolium chloride ([HL ]
3
[
14]
1
[15]
Cl),
1,3‐dibenzylbenzimidazolium bromide ([HL ]Br),
1.90 mmol). The reaction mixture was stirred for 3 h at room
temperature, during which time the mixture became a homo-
geneous brown‐red solution. The reaction solution was fil-
tered and evaporated to dryness. The residue was extracted
with THF (3 × 10.0 ml), and crystallized from THF–hexane
solution at 0 °C. The product 2 was precipitated as brown‐
red crystals in a yield of 88% (1.12 g), m.p. 180 °C. Anal.
Calcd for C H Br FeN (%): C, 37.38; H, 2.84; N, 4.15.
2
[15]
1,3‐dibutylbenzimidazolium bromide ([HL ]Br),
1,3‐bis
3
[15]
(diphenylmethyl)benzimidazolium bromide ([HL ]Br)
2
[14]
and [HL ][FeCl ]
were prepared using published
4
methods. Elemental analysis was performed by direct com-
bustion with a Carlo‐Erba EA‐1110 instrument. NMR spectra
were measured with a Varian Unity INOVA 400 or VNMRS
300 MHz spectrometer at 25 °C. Melting points were deter-
21
19
4
2
mined with a Diamond DSC (PerkinElmer) using powder
Found (%): C, 37.36; H, 2.84; N, 4.15. MS (ESI+): m/z
−
1
+
−1
samples under nitrogen atmosphere (50 ml min ). The sys-
299.15 [C H N ] (100%). Raman spectrum: 204 cm
21 19 2
−1
−
tem was heated from 50 to 350 °C at 20 °C min . GC
([FeBr ] ).
4

LI ET AL.
3
2
2
4
.2.4 | [HL ][FeBr ] (3)
2
Following a procedure similar to that for 2, except that [HL ]
1
Br was used instead of [HL ]Br, the product 3 was precipi-
tated as brown‐red crystals in a yield of 89% (1.02 g), m.p.
1
79 °C. Anal. Calcd for C H Br FeN (%): C, 29.69; H,
15 23 4 2
3
.82; N, 4.62. Found (%): C, 29.66; H, 3.84; N, 4.65. MS
+
(ESI+): m/z 231.19 [C H N ] (100%). Raman spectrum:
15 23 2
04 cm− ([FeBr ] ).
1
−
2
4
3
2
.2.5
|
[HL ][FeBr ] (4)
4
3
Following a procedure similar to that for 2, except that [HL ]
Br was used instead of [HL ]Br, the product 4 was precipi-
1
SCHEME 1 Synthesis of ionic iron(III) complexes 1–4
tated as brown‐red crystals in a yield of 90% (1.48 g), m.p.
1
2
3
smoothly with 1 equiv. of [HL ]Br, [HL ]Br and [HL ]Br,
affording the corresponding ionic iron(III) complexes 2–4
as brown‐red crystals in yields of 88–90%, respectively.
1
83 °C. Anal. Calcd for C H Br FeN (%): C, 47.92; H,
33 27 4 2
3
.29; N, 3.39. Found (%): C, 47.93; H, 3.28; N, 3.37. MS
+
(ESI+): m/z 451.22 [C H N ] (100%). Raman spectrum:
33 27 2
Although FeCl and FeBr are highly hygroscopic, we
04 cm−1 ([FeBr ] ).
−
3
3
2
4
found that the target ionic iron(III) complexes 1–4 were
non‐hygroscopic and existed as air‐stable solids at
room temperature, making them easy to handle. Complexes
–4 showed better solubilities than the corresponding
dialkylbenzimidazolium salts, and all complexes were solu-
ble in THF, Et O and CHCl , sparingly soluble in toluene
2.3 | General procedure for magnesium‐mediated
1
cross‐coupling of aryl phosphates with alkyl bromides
A Schlenk flask was charged with complex 4 (0.02 g,
2
3
0.025 mmol), magnesium turnings (44.4 mg, 1.85 mmol),
and insoluble in hexane.
aryl phosphate (0.5 mmol) and a stirring bar. To this flask
was added a THF solution of LiCl (3.5 ml, 0.5 M) via a
syringe. The mixture was cooled to 0 °C in an ice–water bath.
To this stirred mixture, alkyl bromide (1.75 mmol) was added
at 0 °C via a syringe in a single portion. The reaction mixture
was then stirred for 14 h at 35 °C. The reaction was quenched
by addition of saturated ammonium chloride solution. The
mixture was extracted with ethyl acetate (3 × 3.0 ml) and
dried over Na SO . The GC yield of the desired product
was determined using GC analysis, with n‐hexadecane as an
internal standard. Purification of the crude mixture by flash
column chromatography using petroleum ether (60–90 °C)
as eluent gave the isolated yield of desired coupling product.
The identity of the product was confirmed using NMR spec-
troscopy and TLC.
The structures of 1–4 were determined using elemental
analysis, Raman spectroscopy and ESI‐MS, and X‐ray crys-
tallography for 1 and 4. The Raman spectra of 1–4 confirm
−
the presence of [FeX ] anions. As shown in Figure 1, the
4
−
1
spectrum of 1 shows a single strong peak at 334 cm ,
whereas the spectra of 2–4 show a strong peak in the range
−
1
2
04–205 cm . These data coincide very closely with litera-
[16]
−
−
ture values
for the [FeCl ] and [FeBr ] species, respec-
4 4
2
4
tively. The positive ion ESI‐MS spectra of 1–4 show the
presence of the dialkylbenzimidazolium cation, and in all
cases a peak with an intensity of almost 100% indicative of
the dialkybenzimidazolium cation is observed. Thus, the
characterization results confirm that complexes 1–4 existed
n
1
as [HL ][FeX ] (X = Cl and Br). Unfortunately, the H
4
NMR spectra of the paramagnetic complexes exhibited broad
shifted peaks and yielded little structural information.
Single crystals of complexes 1 and 4 suitable for an X‐ray
crystal structure determination were obtained from THF–hex-
ane solution (1) or ethanol–hexane solution (4) at −10 °C.
3
3
|
RESULTS AND DISCUSSION
.1 | Synthesis and characterization of ionic iron(III)
complexes
The synthetic routes to target iron(III) complexes 1–4 are
shown in Scheme 1. The target ionic iron(III) complex 1
could be prepared in 87% yield by direct reaction of anhy-
1
drous FeCl with 1 equiv. of [HL ]Cl in THF at room temper-
3
2
ature, in a manner similar to that reported for [HL ]
[
14]
[FeCl4].
Further investigation revealed that complex 1
could also be easily prepared by a similar reaction using inex-
[10]
pensive FeCl ⋅6H O as a starting material.
After workup,
3
2
complex 1 was isolated as yellow‐green crystals in 85% yield.
Under similar reaction conditions, anhydrous FeBr reacted
FIGURE 1 Raman spectra of 1 (left) and 2–4 (right)
3

4
LI ET AL.
The crystallographic data, and selected bond lengths and
angles for complexes 1 and 4 are listed in Tables 1 and 2,
respectively. The molecular structures of the complexes are
shown in Figure 2.
Over the past decade, structural data for ionic iron(III)
complexes containing imidazolium cations have been
[
12,14]
rarely reported,
despite the increasing attention such
iron(III) complexes have received for a wide range of
[
17]
applications, such as magnetic materials
catalysts.
and green
[
17,18]
To date, only two papers show structural
data for 1d0i,a14lk] ylbenzimidazolium‐based ionic iron(III)
FIGURE 2 Molecular structures of 1 (left) and 4 (right) with thermal
ellipsoids at the 30% probability level. Hydrogen atoms have been omitted
for clarity
[
complexes.
As shown in Figure 2, each of the two molecular struc-
n +
tures contains one dialkylbenzimidazolium [HL ] cation
and one [FeX4]− anion. The structure of the [HL ] or
1 +
3
+
[
HL ] cations does not change noticeably after reaction of
the corresponding dialkylbenzimiazolium salt with the iron
III) salt. These findings agree with previous reports on
TABLE 1 X‐ray crystallographic data for 1 and 4
(
1
4
[19]
In both [FeX4]− anions,
dialkylbenzimidazolium salts.
Formula
Formula weight
Temperature (K)
Radiation used
Crystal system
Space group
Unit cell dimensions
a (Å)
C
21
H
19Cl
4
FeN
2
C
33
H
27Br
4
FeN
2
each iron atom is coordinated by four halogen atoms in a
slightly distorted tetrahedral geometry with the same mean
X–Fe–X angle of 109.5° for 1 and 4. This angle is close to
497.03
223(2)
Mo Kα
827.06
223(2)
Mo Kα
[
14]
the ideal tetrahedral angle of −50°. The Fe─Cl and Fe─Br
bond lengths are found to lie in the ranges 2.184(2)–2.2022
(19) and 2.306(2)‐2.3468(16) Å, respectively. The mean
Fe─Cl bond length is 2.188 Å in 1, and that of Fe─Br is
2.335 Å in 4. These values agree with those of previous
Monoclinic
P2 /c
Orthorhombic
1
Pnma
7.7541(17)
15.684(4)
18.876(5)
98.040(4)
2273.1(9)
4
16.715(5)
18.311(5)
10.732(3)
90
−
[10,14]
b (Å)
reports on [FeX ] anions.
The structure of 1 indicates
4
that the [HL ] cation and the [FeCl ]− anion are held in
1 +
c (Å)
4
β (°)
place by extensive C─H⋅⋅⋅X interactions (with range less than
3
−
V (Å )
3284.7(16)
4
3 Å). The [FeCl ] anion forms three hydrogen bonds with
4
Z
acidic hydrogen atoms at the C1‐position of the benzimid-
azole ring and two benzylic hydrogen atoms belonging to
two different dialkylbenzimidazolium cations, respectively,
at distances between 2.78 and 2.83 Å. To our surprise, we
observed no typical hydrogen bonding interactions in 4,
which is similar to that found in a bulky imidazolium cat-
3
D
c
(g cm−
)
1.452
1.672
μ (mm−
1
)
1.143
5.349
F(000)
1012
1620
θ range (°)
3.02–27.48
10 817
3.17–27.45
13 824
Reflection collected
[
10]
Independent reflections, Rint
5141, 0.0349
1.197
3850, 0.0597
1.045
ion‐based analogue.
2
Goodness‐of‐fit on F
R
1
, wR
2
[I > 2σ(I)]
0.0763, 0.2016
0.1566, 0.2340
0.0779, 0.2251
0.1516, 0.2696
R
1
, wR
2
(all data)
3.2 | Magnesium‐mediated cross‐coupling reaction
The catalytic performances of complexes 1–4 were then
investigated in the reaction of aryl phosphate 1a with primary
bromide 2a in the presence of stoichiometric magnesium
TABLE 2 Selected bond lengths (Å) and angles (°) for 1 and 4
1
4
[
20]
turnings and LiCl.
in Table 3. Complexes 1–4 and a related analogue, [HL ]
FeCl ], are all capable of activating the aromatic C─O bond
The results obtained are summarized
Fe(1)–Cl(1)
2.184(2)
2.2022(19)
2.178(2)
Fe(1)–Br(1)
2.306(2)
2.341(2)
2
Fe(1)–Cl(2)
Fe(1)–Br(2)
[
4
Fe(1)–Cl(3)
Fe(1)–Br(3)
2.3468(16)
2.3468(16)
108.52(9)
109.05(6)
109.05(6)
109.59(6)
109.59(6)
110.99(10)
of 1‐naphthyl diethylphosphate to give the desired product
aa in 80–94% yields (entries 1–5), along with naphthalene
Fe(1)–Cl(4)
2.1871(18)
109.48(9)
109.26(10)
110.25(9)
107.63(8)
108.28(7)
111.87(8)
Fe(1)–Br(3)′
3
Cl(1)–Fe(1)–Cl(2)
Cl(1)–Fe(1)–Cl(3)
Cl(1)–Fe(1)–Cl(4)
Cl(2)–Fe(1)–Cl(3)
Cl(2)–Fe(1)–Cl(4)
Cl(3)–Fe(1)–Cl(4)
Br(1)–Fe(1)–Br(2)
Br(1)–Fe(1)–Br(3)
Br(1)–Fe(1)–Br(3)′
Br(2)–Fe(1)–Br(3)
Br(2)–Fe(1)–Br(3)′
Br(3)–Fe(1)–Br(3)′
[21]
by‐products in 20–5% yields.
Among these complexes, 4
containing the bulkier N‐diphenylmethyl group is the most
active and affords 3aa in 94% yield at 35 °C. Small differ-
2
ences in activity between complexes 1–3 and [HL ][FeCl ]
4
−
−
suggest that replacing the [FeCl ] anion with [FeBr ] gives
4
4
slightly better catalytic performance. It is noteworthy that

LI ET AL.
5
TABLE 3 Iron‐catalyzed magnesium‐mediated cross‐coupling of 1‐
TABLE 4 Magnesium‐mediated cross‐couplings of aryl phosphates with
alkyl bromides
a
a
naphthyl diethylphosphate and n‐C
6
H13Br
Entry
[Fe] (5 mol%)
Conversion (%)
Yield of 3aa (%)
1
2
3
4
1
2
3
99
99
99
99
99
99
99
99
99
0
80
82
86
b
c
4
94 (81 ; 85 )
d
2
5
[HL ][FeCl
4
]
77
73
79
65
70
0
e
4
6
[H
3
L ][FeCl
4
]
e
4
7
[H
3
L ][FeBr
4
]
8
9
FeCl
3
FeBr
3
3
1
0
[HL ]Cl
a
Reaction conditions: 1‐naphthyl diethylphosphate (0.5 mmol), n‐C
6
H13Br
(1.75 mmol), magnesium turnings (1.85 mmol), LiCl (1.75 mmol), THF, 35 °C,
1
4 h, GC yields using n‐hexadecane as internal standard; an average of two runs.
b
2
5°C.
c
4
5°C.
d
[14c]
Wang et al.
.
e
4 +
a
[H
3
L ] = 1,3‐bis(3,5‐di‐tert‐butyl‐2‐hydroxybenzyl)benzimidazolium cation.
Reaction conditions: 4 (5 mol%), aryl phosphates (0.5 mmol), RBr (1.75 mmol),
magnesium turnings (1.85 mmol), LiCl (1.75 mmol), THF, 35 °C, 14 h, isolated
yields.
b
RBr (2.0 mmol).
related ionic iron(III) complexes containing bis(phenol)‐func-
c
4
Branched/linear ratio in parentheses.
tionalized benzimidazolium cations, i.e. [HL ][FeCl ] and
4
4
[
HL ][FeBr ], afford the desired product in lower yields
4
(
entries 6 and 7). Low yields occur even if the two complexes
(3al) and 57% (3 am) yields. By comparison, 2‐butyl bro-
mide provides the desired 3an in a low 30% yield with mod-
erate branch selectivity. However, our attempt to extend the
substrate to an isopropyl bromide was unsuccessful (3ao).
This transformation has been previously reported to be diffi-
show very high catalytic activities for the cross‐coupling of
aryl Grignard reagents with alkyl bromides and chlorides.
Under the same reaction conditions, FeCl or FeBr alone
afford the products in low yield (entries 8 and 9), whereas
[
10]
3
3
3
[7]
the dialkylbenzimidazolium salt, i.e. [HL ]Br, shows no cata-
cult, and, to date, only two papers have described the use of
[
7,9]
lytic activity (entry 10). These results suggest that the
dialkylbenzimidazolium cations present in 1–4, particularly
those having sterically bulky N‐substituent groups, play a
key role during the catalytic process. This agrees with the
general idea that a sterically bulky N‐heterocyclic carbene
ligand, generated in situ, can facilitate reductive elimination
to form the desired coupling products and regenerate the cat-
isopropyl Grignard reagents
in iron‐catalyzed cross‐cou-
plings with C─O electrophiles. The scope of our methodol-
ogy also included a variety of aryl phosphates that were
compatible with the cross‐coupling reaction. For example,
2‐naphthyl phosphate also couples smoothly with n‐octyl
and n‐hexyl bromides, to afford the corresponding desired
products in 88% (3BC) and 86% (3ba) yields, respectively.
A strongly electron‐donating OMe group on the aryl ring of
a phosphate does not greatly affect the product yield (3ca);
however, a 2‐pyridyl group leads to a 35% yield of the
coupled product (3da). In addition, 4‐biphenyl phosphate, a
rarely used C─O electrophile, also gives a moderate yield
of the desired product (3ea) after increasing the amount
of n‐hexyl bromide in the reaction. Under the same re-
action conditions, less reactive non‐fused phenyl substrates,
such as 3‐trifluoromethylphenyl, 3‐methoxyphenyl and 4‐
trifluoromethylphenyl phosphates, are capable of giving the
desired products in 30–52% yields (3fa, 3ga and 3 ha). These
results indicate the broad generality of the reaction in terms
of substrate scope.
[
22]
alytic active species.
Encouraged by these results, we screened various sub-
strates using complex 4 as a catalyst, as summarized in
Table 4. The reaction of primary alkyl bromides, having var-
ious chain lengths, proceeds efficiently to give the corre-
sponding products in good to excellent yields (3aa–3ae).
Notably, steric bulkiness at the β‐position of the alkyl spacer
in the alkyl bromides is tolerated (3af and 3ag). Furthermore,
functional groups in the alkyl bromides such as phenyl (3ah),
vinyl (3ai), furan‐2‐yl (3aj) and ether and ketal groups (3ak)
also survive the reaction. Secondary cyclic bromides, i.e.
cyclohexyl and cyclopentyl bromide, couple somewhat
slowly to give the corresponding coupling products in 62%

6
LI ET AL.
4
|
CONCLUSIONS
Ed. 2013, 52, 13071. j) D. Gärtner, A. L. Stein, S. Grupe, J. Arp, A. J. von
Wangelin, Angew. Chem. Int. Ed. 2015, 54, 10545.
[
8] S. Protti, M. Fagnoni, Chem. Commun. 2008, 3611.
A family of easy‐to‐handle ionic iron(III) complexes contain-
ing a dialkylbenzimidazolium cation (1–4) were synthesized
by simple methods. We used these complexes as efficient
catalysts for magnesium‐mediated cross‐couplings of aryl
phosphates with primary and secondary alkyl bromides bear-
ing β‐hydrogens under mild conditions. Thus, this work
shows the potential of using aryl phosphates as electrophiles
in iron‐catalyzed C─C bond formation reactions. Because
dialkylbenzimidazolium cations bearing different N‐substitu-
ents are readily available, the present study also provides an
alternative strategy for the development of highly active, yet
robust iron catalysts. Further studies are currently underway
in our laboratory, directed towards understanding the struc-
ture–reactivity relationships of these iron(III)‐based com-
plexes and developing their scope in other cross‐coupling
reactions.
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ACKNOWLEDGMENTS
We are grateful to the National Natural Science Foundation
of China (grants 21172164 and 21472134), the Key Labora-
tory of Organic Chemistry of Jiangsu Province and the
Priority Academic Program Development of Jiangsu Higher
Education Institutions (PAPD) for financial support.
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1 nor 4 since the color of these complexes changes to black during the cat-
−
alytic reaction. In fact, the [FeX
4
]
anion does not remain, which can be

LI ET AL.
7
evident from the Raman spectrum of the reaction solution since the clean
−
[
4
FeX ] peak disappears. It is generally accepted that the actual active spe-
How to cite this article: Li, Z., Lu, B., Sun, H., Shen,
Q., and Zhang, Y. (2016), Ionic iron(III) complexes
bearing a dialkylbenzimidazolium cation: Efficient cat-
alysts for magnesium‐mediated cross‐couplings of aryl
phosphates with alkyl bromides, Appl Organometal
Chem, doi: 10.1002/aoc.3671
cies should be low‐valent iron species. Meanwhile the NHC ligand could be
formed in situ via the deprotonation of an imidazolium salt by various
bases; see ref. [4] and
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